US 2004/0164616-A1 (JP 2004-260908A) proposes methods for controlling a vehicle electrical system that controls a power cost of a vehicle electrical system, thereby to enhance fuel cost improvement. This control method of an electrical system of a vehicle (vehicle electrical system) is carried out in two steps. In one step, the method obtains or calculates information of power costs each coinciding with a cost per unit amount of electric power of respective one of power energy supply sources, such as the source of engine power generation and regenerative power generation. In accordance with the information thus acquired, in the other step, the method adjusts, for example, power supply proportions of the respective power energy supply sources and receive power proportions or allocations of respective electrical loads or a battery to reduce the respective power energy costs.
In many cases, the power cost varies with variations in a supply amount of power. This variation of the power cost is not taken into account in the above control method. Thus, the average power cost of the entire electrical system may not always be minimized, so that effective fuel consumption improvement cannot be achieved.
Further, according to the above control method, characteristics such as response characteristics of the respective power energy supply sources are not taken into account. For example, a necessary consumption amount of power sharply varies. When the response of a power energy supply source with respect to a power generation command is slow, there will occur excessive power generation or deficient power generation. Consequently, the supply voltage from a power bus to electric loads becomes unstable.